The invention relates to a load lifting and transport device with adjustable buoyancy for operations at sea, said device consisting of two parallel groups of floats connected at one of their ends, each group comprising a horizontal float to which vertical floats are fixed by one of their ends, one end of one group being connected to one end of the other group by a horizontal float and distance elements. The invention likewise deals with a method of using said device.
Operations at sea, and particularly the construction or the maintenance of oil drilling platforms, necessitate powerful transport and lifting means employing barges and barge cranes, the limited capacities of which present problems demanding complex and onerous solutions.
French Pat. No. 2,474,992 describes a device with adjustable buoyancy permitting the lifting to considerable heights of loads beyond the capacity of the available barge cranes. The device assumes the shape of a U consisting of two parallel groups of floats connected at one of their ends by a float with horizontal axis and by partly demountable distance elements. The horizontal and vertical floats are ballastable. The vertical floats of one group are connected at their emergent end by distance elements forming a longitudinal beam. The load is placed between the two longitudinal beams, whilst the pumps and the command station are arranged on the distance elements forming a transverse beam at the end of the groups of floats.
Such a device is used, inter alia, for positioning decks on the columns, during the construction or the dismantling of platforms. Generally, the deck is brought up to the site on barges, from which it is taken by the device with adjustable buoyancy which is brought up above the columns on each side of the latter. After ballasting, the deck comes to rest upon the columns.
The device described, and incidentally any other lifting means, being applied to the edges of the deck or of the structural element, subjects the latter to forces which necessitate a reinforcement of the structure in order to prevent deformations. Generally, the structure of a deck is designed to rest on four or more points located within the perimeter of the deck, which transmit all the forces to the columns. The distribution of the bending moments in the structure of the deck corresponds approximately to that of a beam supported at two non-extreme points. In the case of lifting a fully equipped deck by its edges, the distribution of the bending moments corresponds to that of a beam supported at its ends. The realisation of a structure capable of fulfilling both conditions involves a not inconsiderable increase in production costs, due to the increase in the weight of the deck itself.